In semiconductor production environments dispersive spectroreflectometry is used as a non-destructive analyzing method of thin layer systems. An incident radiation beam reflects from a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation includes multiple frequency components or is monochromatic with a time-varying frequency. The reflected radiation is analyzed at a plurality of measuring frequencies, such that a reflectance spectrum is obtained that illustrates the frequency dependence of the intensity of the reflected radiation.
By analyzing the obtained reflectance spectrum the thickness of thin layers in a multiple layer system covering a semiconductor wafer can be determined respectively through model-based algorithms. The model-based algorithms typically use a multiparameter analysis routine to extract the layer parameters. The analysis routine is a fitting method that fits the reflectance spectrum being obtained by measuring with a further, calculated reflectance spectrum that is obtained by calculating the respective values for a model having equivalent model parameters such as film thickness, refractive index and graded transition-profile thickness. The analysis varies the model parameters until the reflectance spectra obtained by calculating and measuring match.
Further Fourier-transform infrared (FTIR) reflectance-spectroscopy methods have been developed as alternative metrology tool for characterizing layer systems on a semiconductor wafer. A Fourier-transform infrared apparatus bases on a scanning Michelson interferometer, which allows the simultaneous measurement of multiple wavelengths. A beamsplitter separates an initial radiation beam into two beams. The first beam has a fixed path length, while the path length of the second beam is periodically varied. The two beams are then recombined such that interference occurs between the beams according to their optical path difference. In this way, an interferogram is obtained that plots the respective radiation intensity against the mirror position, which is related to the optical path difference. Then a Fourier transform of the interferogram is performed, wherein the reflectance spectrum is obtained, which is then analyzed according to the yet discussed various model-based analyzing methods.
According to ellipsometric methods, the incident radiation beam has a known polarization state and the polarization state of the reflected radiation is analyzed to determine properties of the sample.
For patterned layer systems having a 3D-topography the model-based fitting algorithms become more complicated. The layer parameters and the simulated 3D-topography obtained from the model-based fitting algorithm do not always match well with the actual layer system. Especially for a substrate having a 3D-topography with high aspect ratio trenches with partially rough inner sidewalls, the model-based fitting algorithms often render insufficient results.